Degraded population inversion (PI) at elevated temperature, regarded as an important temperature degradation factor in terahertz quantum cascade lasers (THz QCL), has hindered the widespread use of these devices. Herein, the mechanism of the temperature degradation of PI is investigated microscopically. It is demonstrated that the limited extraction efficiency of the extraction system dominates the decrease of PI at elevated temperatures. To be specific, the increased temperature brings about intense thermally activated longitudinal optical phonon scattering, leading to large amounts of electrons scattering to lower level state. In this case, the resonant-phonon extraction system is incapable of depleting all the electrons from lower level states. So even though the resonant-tunneling injection seems efficient enough to compensate the electron runoff at the upper state, the electron density at lower level state increases and the overall PI turns out lower. In addition, it is found that strong electron-ionized donor separation at high temperature can induce level misalignment, which can stagger the optimal conditions of injection and extraction. Also, the extraction efficiency gets lower as the extraction system requires accurate coupling between several energy levels.

中文翻译：

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太赫兹量子级联激光器中以提取为主的群体反转温度退化

高温下退化的粒子数反转（PI）被认为是太赫兹量子级联激光器（THz QCL）中重要的温度退化因素，阻碍了这些器件的广泛使用。在此，从微观上研究了 PI 的温度降解机理。结果表明，萃取系统有限的萃取效率主导了高温下 PI 的降低。具体而言，升高的温度会导致强烈的热激活纵向光学声子散射，导致大量电子散射到低能级状态。在这种情况下，共振声子提取系统无法耗尽低能级状态的所有电子。因此，即使共振隧道注入似乎足以补偿高能级的电子径流，但低能级的电子密度会增加，整体 PI 会降低。此外，发现高温下的强电子电离供体分离会导致能级错位，这可能会错开注入和提取的最佳条件。此外，由于提取系统需要多个能级之间的精确耦合，因此提取效率会降低。